Newly legislated exhaust emission standards for heavy duty diesel engines have revived interest in the use of alternate fuels. Fuel substitution is one possible means of reducing emissions sufficiently to comply with the new standards. Two gaseous fuels, compressed natural gas (CNG) and hydrogen, are among the alternative fuels having the potential to reduce emissions. We have now designed a fast-acting injector which, among a number of different uses, can be employed for timed port injection of CNG into a two-stroke diesel engine (with minor modifications for hydrogen). The CNG is injected into the cylinders through the cylinder liner ports from approximately bottom dead center (BDC) to 60.degree. after BDC. At an operating speed of 2100 RPM, this allows approximately 5 ms to inject the fuel. Our major design target for this particular application of the device was the delivery of an average CNG flowrate of 6.35 Kg/hr (14 lbm/hr) at injection durations of 5.0 ms and a frequency of 2100 injections per minute. We stress, however, that this unit can be applied in a general way for the metering and timing of gaseous fuel to any engine, whether 2-stroke or 4-stroke, and regardless of where, in the cylinder, the gaseous fuel is injected.